ABSTRACT The prevalence of overweight and obesity in youth with serious emotional disturbance (SED) exceeds the epidemic levels of overweight and obesity for children in the general population. Associated cardiovascular (CVD) risk factors such as glucose intolerance, hypertension and lipid abnormalities frequently already exist in obese children. Thus, focusing on overweight/obesity in youth with SED is critical to prevent their premature CVD. In 2015 the American Heart Association called attention to mental illnesses in youth as important risk conditions for early CVD and declared the need for transformational change in screening and management of overweight and obesity. Despite this, studies of healthy weight interventions in youth with SED are noticeably absent, and interventions will likely need to be tailored for this population who frequently have impulsivity, oppositional behavior and/or mood symptoms. In the general population, successful interventions targeting youth obesity often include active parental participation. Community mental health programs serving youth typically engage parents as part of routine treatment and therefore provide an unappreciated but potentially important opportunity to address overweight and obesity. The objective of this R01 application is to design and rigorously test an innovative, scalable healthy weight intervention in youth with SED that incorporates parental engagement and provider support. Our research team has extensive experience spanning child mental health, weight management and physical activity interventions in youth, and in adults with serious mental illness and in the general population. We propose the CHAMPION Trial, a two-arm trial partnering with community mental health organizations in Maryland. We will perform population-based screening to identify youth with overweight and obesity, and recruit and enroll 120 ages 8-18 years with SED with a BMI at or above the 85th%tile. We will stratify by sex and site and randomly assign participants to receive a) usual care plus educational materials on healthy weight, or b) the 12-month CHAMPION intervention with 1) a health coach, a trained mental health program employee, who provides healthy lifestyle counseling and support for weight management, a healthy diet and increased physical activity incorporating techniques to engage both the youth and parents; and 2) support from health providers who will provide basic weight management messages and encouragement for behavior change. The intervention's first 6 months will include frequent contacts, followed by a 6-month maintenance phase. The primary outcome will be decreased BMI z-score compared to control at 12 months; secondary outcomes include waist circumference, blood pressure, glucose, lipid levels, quality of life, and psychiatric symptoms at 6 and 12 mo. Antecedents of CVD begin early in youth, and those with SED are at particularly high risk. Breaking this cascade of risk factors leading to premature CVD will require capitalizing on the window of opportunity to intervene on overweight and obesity in youth with SED. The CHAMPION Trial will test an innovative, yet practical approach that could be readily incorporated into overall care of youth with SED.